A nesosilicate mineral, Al2SiO4(OH,F)2; sp. gr. 3.5–3.6; hardness 8; orthorhombic; colourless, pale yellow, pale blue, yellowish, or sometimes pink; often transparent; vitreous lustre; crystals are prismatic and often bipyramidal with the vertical faces striated, but it can also be massive and granular; cleavage perfect basal {001}; typically occurs in granite pegmatites, rhyolite, and quartz veins, and extensively as an accessory mineral in granites, associated with fluorite, tourmaline, beryl, and cassiterite, also in alluvial deposits. It is associated with pneumatolytic action (see pneumatolysis) and is a constituent of greisen. The original cairngorms (see quartz) were topaz crystals. It is named after Topazos Island in the Red Sea.